Flux-Grown Centimeter-Sized Sodium Niobate−Barium Titanate Solid Solution Single Crystals: Crystal Structure and Physical Properties

Centimeter-sized pale-yellow 0.91NaNbO3-0.09BaTiO3 (NN-9BT) single crystals were successfully grown by the flux method near the orthorhombic−tetragonal phase boundary of the NaNbO3−BaTiO3 system. The as-grown crystals display intergrowing grain features, and the maximum grain size is on the centimeter scale. Room-temperature X-ray diffraction revealed a {100}C-oriented orthorhombic structure dominated by the P21ma phase. At room temperature, the crystals exhibited a remanent polarization (Pr) of 10 μC/cm2 and a coercive field (Ec) of 32 kV/cm. The strain-electric field (S−E) response displayed an antisymmetric profile, which became increasingly pronounced with increasing temperature. The in situ piezoelectric coefficient (d33) reached a maximum value of approximately 160 pC/N at 180−190 °C before decreasing sharply near 220 °C. Temperature-dependent X-ray diffraction and Raman spectroscopy revealed a stepwise structural evolution from P21ma to Pbnm(1), Pbnm(2), and finally P4/mbm. The transition from Pbnm(1) to Pbnm(2) involved not only doubling of the b-axis but also a change in the rotational modes of the Nb/TiO6 octahedra. Impedance spectroscopy yielded an activation energy of approximately 0.986 eV, suggesting a defect-assisted thermally activated conduction mechanism.